INFRARED SPECTRA OF (CO₂)₂-X, CO₂-X₂, AND CO₂-X₃, X = CO OR N₂

Abstract

We previously observed the spectrum of (CO$_2$)$_2$-CO in the region of the CO fundamental band (2150 \\wn).\\footnote{A.J. Barclay, A.R.W. McKellar, and N. Moazzen-Ahmadi, \\textit{Chemical Physics Letters} \\textbf{677}, 127 (2017).} Its structure resembles that of a CO$_2$ dimer (planar slipped-parallel) with a CO monomer aligned along the dimer $C_2$ symmetry axis in a C-bonded configuration, giving a trimer with $C_2$ symmetry. We have now observed three bands of the same species in the CO$_2$ \\nub{3} region. Two of these bands are fundamentals, one $a/c$-type and the other $b$-type. The presence of this $b$-type band establishes that the CO$_2$ dimer subunit within the trimer is actually not planar. The third band is a $b$-type combination band near 2376 \\wn.\n\nA new $c$-type fundamental near 2349.5 \\wn and an $a$-type combination band near 2365.2 \\wn are assigned to CO$_2$-(CO)$_2$. The structure has two C-bonded CO monomers in equivalent positions around the CO$_2$ 'equator', giving a $C_{2v}$ geometry analogous to that of CO$_2$-Ar$_2$. A similar fundamental is observed for CO$_2$-(N$_2$)$_2$ near 2350.0 \\wn. \n\nThe weak parallel band of a symmetric or near-symmetric rotor near 2349.2 \\wn is assigned to CO$_2$-(CO)$_3$, together with an analogous band of CO$_2$-(N$_2$)$_3$ near 2349.8 \\wn. We think that these tetramers are accidental symmetric rotors having $C_s$ symmetry. They are obtained by taking the $C_{2v}$ trimers from the previous paragraph and adding a third CO or N$_2$ to the 'side' of the first two, in the plane bisecting them.